(1) Field of the Invention
This invention relates to a semiconductor devices and in particular to an MIS type semiconductor device comprising a drain region of improved structure.
(2) Description of the Prior Art
Recently, the so-called LDD (lightly doped drain) structure of the drain region has been developed. This is a two-layer structure consisting of lightly doped and heavily doped impurity diffusion regions, and was developed as a means of promoting a high degree of circuit integration of MIS type semiconductor devices (e.g. MOS type semiconductor integrated circuits), and of increasing their breakdown voltage. This is achieved by lowering the strength of the high electric field set up in the vicinity of the drain region as a result of the miniaturization of such transistors.
However, while the lightly doped impurity diffusion region suppresses formation of hot carriers by lowering the electric field near the drain junction, the surface of the lightly doped impurity region easily becomes depleted by the electric field set up in the insulating films such as the gate insulating film by the action of the surface state or the trapped charges induced by hot carriers. This in turn results in an increase in the parasitic resistance due to the lightly doped impurity region, with the result that the current drive capacity of transistors of LDD structure undergoes a characteristic deterioration. The drain electric field lowering effect in the LDD structure increases with decreasing concentration of the dopant in the lightly doped impurity diffusion region, but the aforementioned characteristic deterioration also increases with decreasing concentration of the dopant and this produces the problem that the range within which the dopant concentration of the lightly doped impurity region can be selected is restricted by these mutually opposed requirements.